Electro-hydraulic braking installations conventionally comprise a service braking system using an external energy source and an emergency braking system using muscle power as its source, these two braking systems being controlled by a brake master cylinder, the actuating pedal of which is situated in the cockpit of the vehicle.
The braking system with an external energy source comprises a generator of brake fluid at high pressure, comprising a hydraulic pump associated with a hydraulic pressure accumulator. Upon a braking action performed by this system, the pressure supplied by the hydraulic pressure accumulator is communicated to the wheel brake cylinders via at least one solenoid valve so that the pressure leaving this solenoid valve has a value which is a function of the travel of the brake pedal and of the force with which this pedal is actuated, or a function of the force with which a handbrake lever is actuated, or alternatively a function of the brake fluid pressure produced using the footbrake pedal or the handbrake lever.
In such operation in service braking mode, the master cylinder is normally isolated from the braking installation of the vehicle by means of a shut-off solenoid valve. This then means that brake fluid cannot flow back from the master cylinder towards the wheel brake cylinders and that its piston cannot move or can move only by a minimum travel. However, for the brake pedal or handbrake lever to have a normal actuating travel, depending on the force with which they are actuated, a device which simulates the brake actuation travel is used.
Such a device that simulates the brake actuation travel is known, for example, from document U.S. Pat. No. 4,462,642. The known brake actuation travel simulator comprises a simulator cylinder with a simulator piston which can move in this cylinder when it is acted upon by the pressure of the brake fluid from the master cylinder, against the action of a spring and which can move inside this cylinder.
In the event of failure of the braking system with an external energy source, for service braking mode, the shut-off solenoid valve is switched to allow the master cylinder to actuate the wheel brake cylinders itself, for an emergency braking operating mode using as its energy source muscle power provided by the driver of the vehicle.
The brake actuation travel simulator device according to the aforementioned document is hydraulically connected to the master cylinder and to the shut-off solenoid valve and communicates, even in emergency braking mode, with the brake master cylinder. The known brake-actuating simulator therefore has the drawback of absorbing a certain amount of brake fluid in emergency braking mode using muscle power, and this needlessly increases the brake pedal travel and detracts from the effectiveness in emergency braking.